The invention relates to the production of hydrogen through oxidation reactions of hydrocarbon or oxygenated hydrocarbon compounds. More specifically, the invention relates to the combining of hydrocarbons and oxygenated hydrocarbons with bases to form hydrogen.
As the supply of fossil fuels steadily dwindles and the deleterious environmental consequences of fossil fuels steadily increase, it is becoming increasingly evident that new fuels and forms of energy are needed. Significant efforts have been undertaken over the years to identify acceptable substitutes for fossil fuels. The desired attributes of a new fuel or energy source include low cost, ample supply, renewability, safety, and environmental compatibility.
The alternatives that are being explored can be divided into three broad categories: nuclear power, solar energy, and chemical fuels. In nuclear power, energy is extracted from the natural decay of radioactive elements. Although large amounts of energy are available from nuclear decay processes, the development of nuclear power has been limited because of concerns over the handling of radioactive elements and the disposal of radioactive waste. The public also worries about the possibility of runaway reactions and core meltdown during the operation of nuclear power plants.
Solar energy offers the promise of tapping the enormous energy reserves contained in the sun. The primary objective in solar energy development is the efficient collection and conversion of the energy contained in sunlight to electricity. The conversion is typically accomplished through photovoltaic devices that absorb and transform the wavelengths of light emitted by the sun. The transformation normally involves the production of electrical charge carriers via a valence band to conduction band absorption process in a semiconductor material. A desirable feature of using semiconductors to convert solar energy to electricity is the absence of pollution and the near zero maintenance requirements. Most solar energy devices are based on silicon and much research activity has been directed at optimizing the sunlight-to-electricity conversion efficiency through the development of better materials and innovative device structures. Although much progress has been made and will continue to be made in solar energy, efficiencies are currently limited to 10-15%.
Chemical fuels are a broad class of energy sources and encompass any substance capable of delivering energy through a chemical reaction. Conventional fossil fuels are included among chemical fuels and deliver energy through combustion reactions. The search for new chemical fuels is focusing on materials that combust cleanly and at less extreme conditions than gasoline and other petroleum based fuels. The objective of achieving clean burning fuels is directed at minimizing or eliminating environmentally undesirable by-products such as CO, CO2 and NOx gases. If reaction conditions less extreme than the high temperatures required in standard internal combustion engines can be found, an opportunity exists for developing simpler and lighter weight engines that run more efficiently. Much of the work on synfuels in the 1970""s and 1980""s focused on developing alternative chemical fuels for combustion engines. Various hydrocarbons and oxygenated hydrocarbon compounds such as methanol have been considered. Although some promising results have been obtained, no alternative has proven sufficiently successful to motivate the costly transition from the current fuels to a new fuel source.
Hydrogen is currently the best prospect for replacing or reducing our dependence on conventional fossil fuels. The strong interest in hydrogen is a consequence of its clean burning properties and abundance. When reacted with oxygen, hydrogen produces only water as a by-product. Hence, hydrogen is an environmentally friendly fuel. Hydrogen is also the most abundant element in the universe and is contained in large amounts in many chemical compounds. Hydrogen therefore is an attractive alternative fuel source.
The realization of hydrogen as an ubiquitous source of energy ultimately depends on its economic feasibility. Economically viable methods for extracting and/or recovering hydrogen from chemical feedstocks, as well as efficient means for storing, transferring, and consuming hydrogen, are needed. The most readily available chemical feedstocks for hydrogen are organic compounds, primarily hydrocarbons and oxygenated hydrocarbons. The most common methods for obtaining hydrogen from hydrocarbons and oxygenated hydrocarbons are dehydrogenation reactions and oxidation reactions. Dehydrogenation reactions produce hydrogen by transforming saturated hydrocarbons to unsaturated hydrocarbons. Reformation reactions are a common type of oxidation reaction and involve the breaking of bonds between hydrogen and other atoms such as carbon, oxygen or nitrogen. Hydrogen atoms released upon bond breakage combine to form the desired diatomic hydrogen molecules. The broken bonds remaining on the feedstock molecules recombine or reform to produce new molecules. The reformation process is formally an oxidation reaction of the feedstock molecules.
Production of hydrogen from hydrocarbon and oxygenated hydrocarbon compounds is frequently accomplished with a steam reformation process. In steam reformation processes, a hydrocarbon or oxygenated hydrocarbon (e.g. methanol) feedstock is contacted with water in a high temperature reactor to produce hydrogen gas (H2) along with carbon monoxide (CO) and/or carbon dioxide (CO2). Representative hydrogen producing steam reformation reactions for a general hydrocarbon (CnHm) and a general alcohol (CpHqOH), are given below:
CnHm+xH2O⇄(m/2+x)H2+yCO2+(nxe2x88x92y)CO
CpHqOH+rH2O⇄(xc2xd(q+1)+r)H2+vCO2+(pxe2x88x92v)CO
The hydrocarbon CnHm can be an alkane, alkene or alkyne and the group CpHq on the general alcohol can be an alkyl, alkenyl, or alkynyl group. Similar reactions can be used to describe the production of hydrogen from other oxygenated hydrocarbons such as aldehydes, ketones, and ethers. The relative amounts of CO2 and CO produced depend on the specific reactant molecule, the amount of water used, and the reaction conditions (e.g. temperature and pressure).
Although the prior art steam reformation processes effectively generate hydrogen, they suffer from several drawbacks. First, the reactions are endothermic at room temperature and therefore require heating. Temperatures of several hundred degrees are needed to realize acceptable reaction rates. These temperatures are costly to provide, impose special requirements on the materials used to construct the reactors, and limit the range of applications. Second, the required high temperatures imply that steam reformation reactions occur in the gas phase. This means that hydrogen must be recovered from a mixture of gases through some means of separation. The separation means adds cost and complexity to the reformation process and make it difficult to obtain perfectly pure hydrogen. Finally, the production of CO2 and/or CO is environmentally undesirable since both gases contribute to the greenhouse effect believed to be responsible for global warming.
It is evident that a need exists for producing hydrogen from organic chemical feedstocks in an efficient, economically feasible, and environmentally friendly way. It would be desirable to have a process for producing hydrogen that is effective closer to room temperature than the current commonly used processes and that avoids or minimizes the production of environmentally harmful gases as by-products. Discovery of an acceptable process for producing hydrogen would greatly advance the cause of achieving a clean-burning economy based on hydrogen. Convenient access to hydrogen fuel, coupled to efficient technologies such as fuel cells for extracting energy from hydrogen, offers the potential to greatly reduce our current dependence on fossil fuels.
There is disclosed herein a process for producing hydrogen gas from hydrocarbon or oxygenated hydrocarbon compounds. The process comprises the step of combining a hydrocarbon or oxygenated hydrocarbon with a base, optionally in the presence of water and/or a catalyst. The process effects a reaction of hydrocarbon and oxygenated hydrocarbon compounds to produce hydrogen gas. Inclusion of a base introduces hydroxide ions into the reaction mixture. The hydroxide ions initiate reactions of hydrocarbon or oxygenated hydroxide compounds that proceed substantially through the formation of the bicarbonate ion and/or carbonate ion instead of substantially through the formation of the environmentally undesirable gases carbon monoxide and/or carbon dioxide as occurs in the conventional hydrogen production through steam reformation processes. The hydroxide initiated reactions of the present invention permit reaction in the liquid phase and avoid the high temperature or steam conditions typical of prior art hydrogen production processes. Furthermore, in the hydroxide initiated reactions of the present invention, hydrogen is the only gas phase product produced. Consequently, the hydrogen gas produced in present invention is easily recovered and deliverable to hydrogen-based energy or storage devices such as fuel cells or hydrogen storage alloys.
A wide variety of hudrocarbon and oxygenated hydrocarbon compounds are suitable for the present invention including but not limited to alkanes, alkenes, alkynes, alcohols, and aldehydes. Suitable bases include compounds that produce or lead to the production of hydroxide ions in the reaction mixture. Metal hydroxides are the preferred bases. Various catalysts are effective in accelerating the reformation reaction of the present invention. These include, but are not limited to, transition metals, noble metals, metal oxides, and metal hydrides, either supported or unsupported. In a particularly preferred embodiment of the present invention, hydrogen gas is produced from the reaction of methanol with an aqueous potassium hydroxide solution in the presence of a catalyst comprised of platinum supported on carbon.